The present invention relates to an improved motor drive controller. More particularly, the present invention relates to an improved motor drive controller for controlling a variable speed motor.
Variable speed motors are presently in wide use. Typically, a variable speed motor includes a motor drive for varying the speed of the motor, the motor drive having a speed control input which responds to an analog speed signal. As the level of the speed signal varies, the motor drive increases or decreases the speed of the motor. Therefore, an operator can control the motor speed by providing the analog speed signal, at a desired level, to the speed control input of the motor drive.
Some typical motor drives control motor speed by using armature control. As the speed signal at the speed control input increases, the motor drive increases a voltage level applied to armature windings in the motor. The increased armature voltage causes the rotational velocity of the armature to increase. Similarly, as the speed signal is decreased, the motor drive decreases the voltage level applied to the armature windings and the rotational velocity of the armature decreases.
In the baking industry, variable speed motors are used in a variety of machines for processing dough. One such machine is known as a sheeter in which a large ball of dough is placed on a conveyor and is moved through a series of rollers which roll out the ball of dough and spread it into a sheet of dough having generally uniform thickness. The dough can then be processed in any desired manner.
A second machine which processes dough, and which utilizes a variable speed motor, is a cutter. In a cutter, the sheeted dough is placed on a conveyor and moved through a rotating cutter which cuts the dough into any desired form, such as doughnuts. The cut dough is then collected for further processing.
Both sheeting and cutting require variable speed motors because the sheeting and cutting processes can be done at higher or lower speeds depending on factors such as the volume of dough being sheeted or cut and dough consistency. Also, some dough processing machines perform both cutting and sheeting functions. These machines require variable speed motors because, typically, cutting must be performed at lower speeds than sheeting. Therefore, when the operator is using the machine to cut, the machine must be moving at a slower rate than when the operator is using it to perform the sheeting operation.
Nearly all machines suitable for processing dough have various safety interlocks. For example, when a jam occurs in a sheeter, in order to clear the jam, the operator must lift a roller guard to gain access to the rollers. When the roller guard is lifted, a roller guard safety interlock is tripped. Once the safety interlock is tripped, a power interruption mechanism interrupts power to the motor drive and the motor is turned off.
However, the speed signal provided to the motor drive generally stays at the same level regardless of whether a safety interlock is tripped. Therefore, if the power interruption mechanism should fail, the motor drive will continue driving the motor at a speed corresponding to the level of the speed signal.
Therefore, there is a continuing need for a motor drive controller which ensures that, once a safety interlock is tripped, the motor speed is immediately reduced to zero.